2011A&A...529A.150C

The study of water masers at cosmological distances would allow us to investigate the parsec-scale environment around powerful radio sources, to probe the physical conditions of the molecular gas in the inner parsecs of quasars, and to estimate their nuclear engine masses in the early universe. To derive this information, the nature of the maser source needs to be assessed through a detailed investigation of the observational characteristics of the line emission. This will determine whether the emission is from a jet- or disk-maser source. We monitored the maser line in the lensed quasar MG J0414+0534 at z=2.64 with the 300-m Arecibo telescope for ∼15 months to detect possible additional maser components and to measure a potential velocity drift of the lines. In addition, we followed the maser and continuum emissions to reveal significant variations in their flux density and to determine whether correlation or time-lag exists between them. The main maser line profile is complex and can be resolved into a number of broad features with line widths of 30-160km/s. A new maser component was tentatively detected in October 2008, and is redshifted by 470km/s compared to the systemic velocity of the quasar. The line width of the main maser feature increased by a factor of two between the discovery Effelsberg and EVLA observations and the first epoch of the Arecibo monitoring campaign. After correcting for the lens magnification, we find that the total H₂O isotropic luminosity of the maser in MG J0414+0534 is now ∼30000L_☉, making this source the most luminous ever discovered. Both the main line peak and continuum flux densities are surprisingly stable throughout the period of the observations. The integrated flux density instead shows significant variations on monthly time scales, possibly due to changes in the individual velocity components. We place an upper limit on the velocity drift of the peak of the line emission of 2km/s/yr. The large line width of the main maser line and the absence of a clear triple-peak pattern in the maser spectrum of MG J0414+0534 favours the jet-maser scenario. However, the stability of the line and continuum emission, and the presence of the tentative new maser component, which is potentially identified as a high-velocity feature of a rotating disk, seems to partly contradict this interpretation. Sensitive monitoring on a longer time scale and VLBI observations are mandatory for drawing a definite conclusion.